IGMP is a layer 3 protocol, which means that switches (operating at layer 2) should be unaware of it. However, if switches are unaware of IGMP, how can they know what ports to send a multicast packet to?

Is IGMP a layer 2?

IGMP is a network layer (Layer 3) protocol used to establish membership in a Multicast group and can register a router to receive specific Multicast traffic. (Refer to RFC 1112 and RFC 2236 for information on IGMP versions 1 and 2.)

Is IGMP layer 2 or Layer 3?

Essentially, IGMP snooping is a layer 2 optimization for the layer 3 IGMP. IGMP snooping takes place internally on switches and is not a protocol feature. IGMP snooping allows a switch to only forward multicast traffic to the links that have solicited them.